This invention relates to an adjustment means for orienting to a horizontal plane, the structure supporting compass cards immersed in damping fluid of magnetic compasses of the class enclosed in a spherical housing.
U.S. Pat. No. 3,481,045 discloses an example of a spherical compass with double-axis internal gimballing and a member providing damping against roll and pitch supportive of lubber and athwartship lines maintained perpendicular to the compass card. In recent years, location and mounting of such compasses have changed drastically from a true pedestal type binnacle mount, where the compass is installed horizontally, to angled shelf-mounted units and, most recently, to athwartship bulkhead mountings which may be vertical or pitched at some unspecified angle to the horizontal. At present, internally gimballed compasses must be preconstructed to adapt properly to surface mounting at any angle to the horizontal, because in order for the compass card to function properly, it must, when the vessel is at rest, be lying in a horizontal plane. Several presently available spherical compasses, providing double axis gimballing mechanisms having the trunnions of the outer gimball lying in the athwartship plane, additionally give rise to substantial cardanic error when the vessel is sailing at a considerable angle of heel and encounters pitching motion. For example: if the angle of heel is 45.degree., a pitch angle of but 15.degree. will produce a displacement of the forward lubber line relative to the compass card of several degrees; in the extreme, if an angle of heel were 90.degree., any pitch would produce up to 180.degree. of lubber line displacement. If the compass is installed at an angle from horizontal with its outer trunnions in the fore-aft plane, the same error is encountered.